Thin film optical coatings known as interference filters which comprise alternating layers of two or more materials of different indices of refraction are well known to those skilled in the art. Such coatings or films are used to selectively reflect or transmit light radiation from various portions of the electromagnetic radiation spectrum such as ultraviolet, visible and infrared radiation. These films or coatings are used in the lamp industry to coat reflectors and lamp envelopes. One application in which these thin film optical coatings are useful is to improve the illumination efficiency or efficacy of incandescent lamps by reflecting infrared energy emitted by a filament or arc back to the filament or arc while transmitting the visible light portion of the electromagnetic spectrum emitted by the filament. This lowers the amount of electrical energy required to be supplied to the filament to maintain its operating temperature.
In other applications where it is desired to transmit infrared radiation, such filters can reflect the shorter wavelength portions of the spectrum, such as ultraviolet and visible light portions emitted by the filament or arc and transmit primarily the infrared portion in order to provide heat radiation with little or no visible light radiation. Such an application of this latter type would include a typical radiant heater for residential or industrial use where visible radiation emitted by the heater is unwanted.
Such interference filters useful for applications where the filter will be exposed to high temperatures in excess of about 500xc2x0 C. have been made of alternating layers of tantala (tantalum pentoxide Ta2O5) and silica (SiO2), wherein the silica is the low refractive index material and the tantala is the high refractive index material. These interference filters may be obtained by a low pressure chemical vapor deposition (LPCVD) process to produce the film on a suitable substrate as described in U.S. Pat. No. 4,949,005, the contents of which are hereby incorporated by reference.
Improvements in the performance of LPCVD Ta2O5:SiO2 thin film systems have been largely limited by mechanical failure, due to excessive tensile stress in the coating. The limiting number of layers has historically been 46, above which the coatings spall off the substrates. Competitive pressures require the performance of halogen IR films to be improved, with respect to the lumens output per watt consumed, while keeping color quality high.
Previous approaches typically have been to (1) increase the number of layers, (2) switch to a material system with a larger difference in the indices of refraction (n) between the two selected materials, or (3) induce some form of stress relief in some of the layers.
Difficulty with the first approach has typically been with regard to stress. With an increasing number of layers for a given design, the stress (tensile for LPCVD) also increases, ultimately resulting in cohesive failure of the substrate.
Difficulty with the second approach has typically been with regard to finding compatible materials and processes. Relatively few material systems (combinations of high and low index materials) exist which are suited for the intended environment, and also lend themselves to practical processing techniques.
There are also difficulties with the third approach. For example, U.S. Pat. No. 5,680,001, the contents of which is hereby incorporated by reference, describes a method of depositing an adhesive layer between the substrate and the film. These adhesive layers are typically silica doped with either B2O3 or phosphorous pentoxide. These films are hydroscopic, which can lead to detrimental effects on the film.
U.S. Pat. No. 4,949,005 describes a method of annealing tantala/silica structures as a means of relieving stress in the film. This technique is currently in practice for the 46 layer design deposited commercially on halogen filament tubes. The benefits of thermal annealing are supplementary to the invention disclosed presently.
Thus, there is a need to improve the performance of IR reflecting films. The thickness of the alternating layers of tantala and silica are designed such that the visible spectrum is transmitted through the coatings, and the IR energy is reflected, as disclosed in U.S. Pat. No. 5,138,219, the contents of which is hereby incorporated by reference. An increase in performance could be expected simply by increasing the number of layers. However, the internal stresses in the film also increase with the number of layers, for a given design.
An optical interference coating for reflecting infra-red radiation and transmitting visible light comprising alternating layers of high index of refraction material and low index of refraction material, wherein the total number of said layers is greater than 51.